This application claims priority from Japanese 2007-039326, filed 20 Feb. 2007.
1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery, in particular to improvement in high-temperature storage stability.
2. Description of the Related Art
In progress of portable electronic devices such as cellphone, laptop computer and personal digital assistants (PDAs), there is a demand for a high-energy-density secondary battery for use as the main or backup power source in these devices. Lithium-ion secondary batteries, which are higher in voltage and energy density, are attracting attention as a secondary battery suitable for such applications.
In addition, lithium-ion secondary batteries currently used as the power source for the portable electronic devices are mainly nonaqueous electrolyte secondary batteries. Such a nonaqueous electrolyte secondary battery is generally produced by forming electrode assembly by winding spirally or laminating a positive electrode containing a lithium atom-containing composite transition metal oxide such as lithium cobaltate (LiCoO2) as the positive electrode active material and a negative electrode containing for example a carbon material as the negative-electrode active material as they face each other via a separator, followed by enclosing the electrode assembly together with a nonaqueous electrolyte solution in a battery case. For example, a porous film of polyethylene or polypropylene has been used as the separator. As the nonaqueous electrolyte solution, a solution dissolving a solute in a nonaqueous solvent is generally used. Examples of the nonaqueous solvents for use include cyclic and linear carbonate esters and the like. Examples of the solutes for use include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4) and the like.
For improvement of battery characteristics, there have been many studies aimed at improvement of the components for the nonaqueous electrolyte secondary battery: positive electrode, negative electrode, separator, nonaqueous electrolyte solution, and others.
Known as an example of the nonaqueous electrolyte secondary battery having improved positive and negative electrodes is a nonaqueous electrolyte secondary battery having a porous protective film in which a coating film containing a resin binder and solid fine particles is formed on the surface of the active material layer of the positive or negative electrode (see, for example, Japanese Patent No. 3371301). Japanese Patent No. 3371301 above discloses that, by forming the porous protective film on the surface of the active material layer of positive or negative electrode, it is possible to prevent separation and re-deposition of the active material particles during production of the battery and thus to prevent internal short circuiting induced by the re-deposited active material particles.
Known as an example of the nonaqueous electrolyte secondary battery having an improved separator is a nonaqueous electrolyte secondary battery having a separator consisting of a layer containing a heat-resistant nitrogen-containing aromatic polymer such as aramide and a ceramic powder, and a laminated porous film made of a thermoplastic resin (see, for example, Japanese Patent No. 3175730). Japanese Patent No. 3175730 above discloses that the porous film melts, leading to shutdown of blocking the current when the battery temperature rises for example by trouble such as overcharge and that, even when the battery temperature rises further, it is possible to prevent generation of large holes in the separator to prevent meltdown thereof, because the layer containing a heat-resistant nitrogen-containing aromatic polymer and a ceramic powder is resistant to melting. Thus, Japanese Patent No. 3175730 discloses the nonaqueous electrolyte secondary battery favorable both in shutdown capability and meltdown resistance and thus higher in safety.
Known as an example of the nonaqueous electrolyte secondary battery having an improved electrolyte solution is a nonaqueous electrolyte secondary battery having a nonaqueous electrolyte solution containing an aromatic compound such as pyrrole. (see, for example, Japanese Unexamined Patent Publication No. 2002-270226). Disclosed in Japanese Unexamined Patent Publication No. 2002-270226 is that use of such an electrolyte solution is effective in providing favorable cycle characteristics, because it leads to generation of a cover layer on the surface of the active material particles, preventing direct contact between the electrode active material and the electrolyte solution and disturbing the reaction between lithium (Li) and the electrolyte solution during charge and discharge.
However, in the nonaqueous electrolyte secondary batteries described in Japanese Patent Nos. 3371301 and 3175730 and Japanese Unexamined Patent Publication No. 2002-270226, the positive-electrode active material layer containing a transition metal oxide releases the transition metal as a metal cation during high-temperature storage. The released metal cation deposits as metal on the negative electrode, often raising the impedance of the negative electrode and causing clogging in the separator. These nonaqueous electrolyte secondary batteries were thus, still unsatisfactory in high-temperature storage stability, because of the deterioration in discharge rate characteristics caused by such troubles after high-temperature storage.